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834                             Appendix II.

T28 f2 a     •
Resilience per unit volume = • — ^ ------- r v


Similarly, taking bolt B, total resilience

Jti»               2

^0*2 /2#                      /2

Resilience per unit volume =      ^ -- r- vol. = "4654^-

XL                                           iL

Resilience per unit vol. A __ 334 _
Resilience per unit vol. B      465              —

And bolt B is more economical for resisting shock, shewing why it
is better to turn down a bolt shank as at p. 402 to the diameter at
bottom of screw thread.

P. 369. Testing Machines. — The Drop-testing Machine
has always been favoured by railway companies for proving rails
and car axles. The apparatus is simple, consisting merely of a
large anvil on which are supports for the beam to be tested, a pair
of upright guides, and a falling weight that can be raised to any
definite height within them. The French railways use a * monkey '
(falling weight) of 440 Ibs. with a drop of u ft. 6 ins., the rail
supports being 3ft. yjins. span, and the anvil weighing 10 tons.
Messrs. Cammell use a weight of i ton, a fall of 20 or 30 ft., and
a rail span of 3 ft. Rigidity and inertia of anvil are of considerable
importance, but the chief difficulty is to attain constant rigidity.
The Pennsylvania railroad has therefore placed its anvil, weighing
17,500 Ibs., upon 12 stiff helical springs, each having two coils of
8" and 5^" diameter respectively, the outer one of i^" steel and
the inner of y£" steel, 9^" long uncompressed and 5^-" com-
pressed. These can support 80,000 Ibs., and exert a constant
resistance whatever the condition of ground. The monkey
weighs 1640 Ibs., its maximum fall being 43 feet, and the supports
are 3 ft. apart for axle-testing. It is specified that the axles shall
not deflect more than 5^-" with the first blow, delivered from a
height of 23!- ft, and shall stand five blows before fracture.

Testing for Hardntss* — Hardness may be defin ed as the resistance
to permanent deformation, often a property of great importance,
and a means of testing hardness (with accuracy is very much to
be desired All hardness tests depend upon making an indenta-
tion in the material by means of a harder substance, but the